1. Technical Field
The present invention relates to a line head module used as an exposure unit in an image forming apparatus, and an exposure apparatus and an image forming apparatus including the line head module.
This application claims the benefit of Japanese Patent Application No. 2005-066799, Japanese Patent Application No. 2005-066800, Japanese Patent Application No. 2005-066801, and Japanese Patent Application No. 2005-066802 filed on Mar. 10, 2005, in the Japan Patent Office, the disclosure of which is incorporated herein in its entirety by reference.
2. Related Art
As an electrophotographic printer, a line printer (image forming apparatus) is known. In this line printer, a charger, a line-shaped printer head (line head), a developing device, a transfer device are arranged in the vicinity of the circumferential surface of a photosensitive drum to be exposed. That is, on the circumferential surface of the photosensitive drum charged by the charger, an electrostatic latent image is formed by performing exposure due to selective light emission of a light emission element provided in the printer head, a toner is fed onto the electrostatic latent image to form a toner image, and the toner image is transferred onto a sheet of paper by the transfer device.
As the light emission element of the printer head, a light emitting diode (LED) is typically used. However, in this case, it is difficult to arrange several thousand light emission points with high precision. Accordingly, recently, an image forming apparatus including, as the printer head, a light emission element array which includes, as the light emission element, an electroluminescence element (EL element), and more particularly, an organic EL element which can arrange light emission points with high precision is suggested (for example, see JP-A-2003-1864).
In a case where the LED or the EL element is used as a light source in the printer head, since one head is assigned to a photosensitive drum for each color, the life span of the light source must increase in order to increase the printable number of the printer. In addition, in order to obtain a high printing speed, the light amount of the light source must increase. However, if the light amount increases, the life span of the organic EL element decreases and thus the printable number decreases.
Moreover, since the definition of the printer head is uniform, over-specification may be generated when a line image is output.